A certain conventional hydraulic accumulator of the described type includes a shell defining a pressure space, and a metallic bellows unit which is disposed within the pressure space and whose one end is fixedly secured to an end wall of the shell. The metallic bellows unit sections the pressure space into an outer chamber serving as a gas chamber, in which a pressurized gas is enclosed, and an inner chamber serving as a liquid chamber communicating with a liquid port formed in the end wall of the shell. This conventional hydraulic accumulator is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-116003.
In the hydraulic accumulator disclosed in the above-identified publication, when the liquid chamber is opened to the atmosphere, the gas pressure enclosed in the gas chamber acts on the outer side of the collapsed metallic bellows unit and the atmospheric pressure acts on the inner side of the collapsed metallic bellows unit, whereby a considerably large differential pressure acts on the metallic bellows unit. Such differential pressure shortens the life of the metallic bellows unit. In order to suppress possible shortening of the life of the metallic bellows unit, this known hydraulic accumulator employs a sealing structure for preventing the atmospheric pressure from acting on the inner side of the bellows portion of the metallic bellows unit.
However, in the above-described sealing structure, there is a fear that a sealing member that is seated on a seal seat when the liquid chamber is opened to the atmosphere may be fixed to the seal seat and be difficult to separate from the seal seat. In this event, there would be a fear that, since the metallic bellows unit does not smoothly expand, the pressure of pressurized liquid may reach a preset upper limit before a predetermined amount of pressurized liquid is accumulated in the liquid chamber. As a result, the hydraulic accumulator may fail to reliably accumulate pressurized liquid.